The present invention relates a fixing device for fixing a developer image formed on a transfer medium, an image forming apparatus with the fixing device, and a method of fixing the developer image.
An image forming apparatus such as a copying machine or a printer has a fixing device for heating and fusing a developer image formed on a transfer sheet, thereby fixing the developer image on the transfer sheet. This fixing device generally has a heating roller incorporating a heating source such as a heater, and a press roller in rolling contact with the heating roller with a predetermined pressure. The transfer sheet formed with the developer image is conveyed through a portion between the heating roller and press roller. During conveyance, the developer image is fixed to the transfer sheet by the heat and pressure respectively applied by the heating roller and press roller.
The fixing device has a cleaning roller for removing contamination attaching to the heating roller and press roller. The cleaning roller is in rolling contact with the outer surface of at least the heating roller and is urged against the heating roller with a predetermined pressure. As the cleaning roller, a felt roller is mainly used. Felt rollers are roughly classified into those impregnated with silicone oil to supply it, and those not using silicone oil. Felt rollers of either type perform cleaning by wiping out contamination on the heating roller.
When the pressure acting on the cleaning roller is large, although the cleaning performance of the cleaning roller increases, the cleaning roller becomes dirty at an early stage to shorten its service life. To the contrary, when the pressure acting on the cleaning roller is small, although the service life prolongs, the cleaning performance of the cleaning roller degrades. Then, contamination on the heating roller and the press roller cannot be removed sufficiently, and undesirably attaches to a transfer sheet.
When the heating roller and the press roller are cleaned, contamination such as the removed toner, paper dust, and the like attach to the felt roller. If a large amount of contamination attaches to the felt roller over a long-term use, for example, at the start of the fixing operation of the fixing device, contamination attaching to the felt roller is sometimes transferred from the felt roller to the heating roller and the press roller. In this case, contamination attaches to the transfer sheet to cause an image defect. Contamination transfer as described above tends to occur particularly when the pressure acting on the cleaning roller is large.